Monoclonal antibody (MoAb) specific to 28.5 kDa tegumental antigen (TA) was used to localize this antigen in various tissues of adult Fasciola gigantica by means of indirect immunofluorescence, immunoperoxidase and immunogold techniques. The indirect immunofluorescence and immunoperoxidase detections revealed that this antigen was concentrated in the tegument particularly in its outer rim, tegumental cells and their processes, epithelial linings of the oral sucker and the proximal part of digestive tract. It was also detected at a moderate concentration in spermatogenic cells in the testes, cells of Mehlis' gland, oocytes within the ovary, and ovum within the egg of adult parasites. At TEM level, the immunogold detection showed deposit of gold particles specifically in G(2) tegumental granules and on the surface membrane. Thus, this antigen is expressed in the tegument and associated structures of adult parasites, and it could be a major component of the G(2) granules which are shown to fuse with the surface membrane and contribute material to replace the casted-off membrane. This process is a part of membrane turnover that prevents the parasite from being attacked by the host immune effector cells.